Biological Sciences Research Highlights

February 2017

Particles Carry Hitchhiking Pollutants Far from Home

New research shows that atmospheric particles can encase a toxic payload, allowing it to travel further than previously thought

How PAHs Travel In the atmosphere, tiny aerosol particles can form a hardened-sap-like shield around the toxic particles, encapsulating them and trapping these toxics, thus providing a mechanistic explanation for their observed atmospheric persistence. The glassy shell gives the toxic payload protection from chemical oxidants that would normally break them down. This allows the particles to be transported much greater distances than previously thought. Researchers at PNNL and their collaborators treated these toxic particles in this new way in their models, and simulations now agree much more closely to observations of these substances around the world. Graphic by Nathan Johnson at PNNL.Enlarge Image.

Combining state-of-the-art atmospheric modeling
with the latest measurement-based findings, researchers at Pacific Northwest
National Laboratory found that toxic particles can last longer and travel much
farther than previous models predicted.

These toxic bits are a class of persistent
organic pollutants called polycyclic aromatic hydrocarbons, a.k.a. PAHs, which are
harmful to human health and ecosystems. Previously, models assumed these hostile
compounds would chemically degrade within a few hours in the atmosphere. But
the models did not match measurements of PAHs taken in more than 300 urban and
rural settings around the globe.

The organic aerosol particles that coat the
toxic hitchhikers are wafted into the atmosphere through emissions from trees
(like those that produce the smell of pine trees), and burning biomass and
fossil fuel to form a semi-solid sap-like casing surrounding and protecting the
particle's payload from breaking down in the atmosphere.

The new insights indicate an estimate of
global lung cancer risk from these pollutants four times higher than previously
thought. The work was published in the Proceedings
of the National Academy of Sciences.

Why it Matters:
Pollutants
from fossil fuel burning, forest fires and biofuel consumption include a mix of
chemicals and organic particles. Some of these particles-PAHs-appear on the
United Nation's list of persistent organic pollutants where they are regulated
as air pollution. In the United States, the Environmental Protection Agency has
identified several PAHs as cancer-causing agents. Previously, scientists
assumed that PAHs degraded in the atmosphere, so that's how they were
represented in the models. However, when they compared the models to the amount
of PAH that is actually measured in the environment, the numbers didn't add up.

Methods: To solve this
mystery, researchers led by Dr. Manish Shrivastava explored the discrepancy between the high amounts measured
in the atmosphere and the model predictions. They changed the models to match
the most current understanding of how particles react and persist. Scientists'
understanding of aerosols has changed in the last five years. Recent experiments led by PNNL coauthor Dr. Alla
Zelenyuk
show that, depending on the conditions, the aerosol coatings can actually be
quite viscous, that is, thick and sticky. When the atmosphere is cool and dry,
the coating becomes more like hardened sap, trapping chemicals such as PAHs. Shielded
by the coating, these PAHs can survive long distances, taking their harmful
health effects to locations far from their origin.

To find out if including the shielding of
PAHs by viscous aerosols would improve global atmosphere models, the scientists
used laboratory experiments to develop a new way of representing PAHs in a global
model. Then they ran the model to simulate PAH concentrations from 2008 to
2010. They compared the simulation results to data from 69 rural sites and 294
urban sites worldwide.

To look more closely at how far the PAHs can
travel while riding shielded on a viscous aerosol, the researchers compared the
model's numbers to PAH concentrations measured at the top of Mount Bachelor by
coauthor Dr. Staci Simonich of Oregon State University, Corvallis. Research
suggests these aerosols might come all the way from the other side of the
Pacific Ocean. The team found the predictions with the new shielded model of
PAHs came in at the same value that Simonich measured on the mountain-four
times higher than the previous model, which was considerably off the mark.

These predictions from models that considered
shielded PAHs were far more accurate than previous predictions which included
unshielded pollutants. To see an overall view of this long-distance travel, the
team compared the old and new models on a world map. Zooming in on sources in
particular regions allowed them to look at how far the protected PAHs could
travel. In all cases, the shielded PAHs traveled across oceans and continents,
whereas in the previous version they barely moved from their country of origin.

"We developed and implemented new
modeling approaches based on laboratory measurements to include shielding of
toxics by organic aerosols in a global atmosphere model that resulted in large
improvements of model predictions," said PNNL scientist Dr. Manish
Shrivastava.
"This work brings together theory, lab experiments and field observations
to show how viscous organic aerosols can largely elevate global human exposure
to toxins by shielding them from chemical degradation in the atmosphere."

What's Next? "We don't yet
know what the implications of more PAH oxidation products over the tropics are for
future human or environmental health risk assessments. We need to better understand
how shielded PAHs might vary depending on the complexity of the aerosol
composition, atmospheric chemical aging of particles, temperature, and relative
humidity," said Shrivastava.

Acknowledgements

Sponsor: This research
was supported by the Laboratory Directed Research and Development Program at
Pacific Northwest National Laboratory; the National Institutes of Health; the
National Institute of Environmental Health Sciences; and National Science
Foundation. A.Z. was supported by the U.S. Department of Energy (DOE), Office
of Science, Office of Basic Energy Sciences.

Facilities:EMSL, the Environmental Molecular Science Laboratory,
a DOE Office of Science user facility sponsored by DOE's Office of Biological
and Environmental Research and located at PNNL and the core facilities of
RECETOX Research Infrastructure, funded by the Ministry of Education, Youth and
Sports of the Czech Republic. The PNNL Institutional Computing program and EMSL
provided computational resources for the model simulations.